Definition
On a rotating propeller, the blade that is moving downward on one side of the propeller disc as it rotates. When the aircraft is flown at a high angle of attack, the descending blade takes a larger bite of air than the blade rising on the opposite side, producing more thrust on that side and yawing the aircraft. On a typical American engine that rotates clockwise as viewed from the cockpit, the descending blade is on the right side of the propeller disc, which is why P-factor yaws the nose left at high angles of attack.
Plain English
It is the propeller blade that is sweeping downward as the prop spins. When the nose is pitched up, this downward-moving blade pushes harder than the one going up, and that uneven push twists the airplane to one side.
Context Anchor
Seen in discussions of propeller turning tendencies, especially asymmetric loading, commonly called P-factor.
Why Pilots Care
It is the primary source of the left yaw that must be countered with right rudder in high angle-of-attack, high-power conditions.
Intuition Check
Do not think of the descending blade as a separate special blade on the propeller. It is whichever propeller blade is moving downward at that moment as the propeller spins.
Example Sentence 1
During the climb after takeoff, the descending blade on the right side of the propeller produces more thrust than the ascending blade on the left, so the pilot adds right rudder to keep the nose straight.
Example Sentence 2
The pilot adds right rudder to offset the yaw created by the descending blade during a maximum-performance takeoff.